1. Field of the Invention
Embodiments of the present invention relate generally to optical communication systems and components and, more particularly, to a liquid crystal-based optical switch.
2. Description of the Related Art
In optical communication systems, it is sometimes necessary to perform 1×2 switching of an optical signal, where an input light beam enters an optical switching device through an input fiber and is directed to one of two output fibers. There are also more complicated optical switches, such as 2×2, 1×N, and N×N optical switches, which are realized by combining several 1×2 optical switches.
One means for redirecting a light beam along different optical paths in optical communications systems is the mechanical optical switch. A mechanical optical switch has a movable optical part, such as a prism, mirror, or segment of optical fiber, which can be positioned to direct a light beam along one or more alternate optical paths. Drawbacks of mechanical optical switches include slow switching speeds and problematic reliability. Therefore, there has been an ongoing effort in the development of non-mechanical switches for use in optical communications systems.
Liquid crystal (LC) based optical switches are known and used in the art for routing optical signals without the disadvantages of mechanical optical switches. When a potential difference is applied across an LC material, the molecular orientation of the liquid crystals in the LC material become aligned in a known direction. Because the molecular orientation of an LC material changes the polarization plane of incident light, the application of a potential difference across a cell containing an LC material may be used to modulate the polarization of polarized light passing through the cell. For example, in a first state, wherein a potential difference of approximately zero V is applied and maintained across the LC cell, linearly polarized light passing therethrough is rotated 90°. In a second state, wherein a predetermined potential difference, e.g., 5 volts, is applied across the LC cell, linearly polarized light passes therethrough essentially unchanged. The light beam may then be directed through an optical steering element, such as a birefringent crystal, which directs the light beam along one of two optical paths based on the polarization state of the light beam.
Unlike the reflective surfaces of mechanical optical switches, an LC-based optical steering device directs light to different paths by controlling allocation of energy to the two linear orthogonal polarization modes, e.g., p- or s-polarization, via voltages applied to the LC cell. By controlling the voltage applied to the LC cell, all light energy can be allocated to one polarization mode and completely blocked to the other mode, or partially allocated to both modes. These controlling voltages provide the potential switch states. However, the voltage tolerance for these states is usually not the same, resulting in a low extinction ratio.
FIG. 1 illustrates the electro-optical response of a twisted-nematic (TN) LC commonly used in the art. The abscissa represents the potential difference, in volts, applied across the LC, and the ordinate represents the corresponding attenuation, in decibels, of light directed through the LC, where 0 dB of attenuation indicates 100% of the incident optical energy is transmitted. In this example, the incident light is initially p-polarized. EO curves 101 and 102 depict the intensity of the s-polarized component and the p-polarized component in the light after p-polarized light passes through the LC cell. For example, at zero volts, the intensity of p-polarized light is around −22 dB, and the intensity of s-polarized light is approximately 0 dB, indicating the LC converts most of the p-polarized light to s-polarized light. At 10 volts, the intensity of p-polarized light is approximately 0 dB and the intensity of s-polarized light is less than −45 dB, indicating the LC converts a very small portion of incident light to s-polarization. For each curve, the magnitude of attenuation is equivalent to the blocking ratio of the LC, which, as shown in FIG. 1, is asymmetrical between s- and p-polarized light.
For example, to allow transmission of incident p-polarized light without changing the polarization thereof, a potential difference of about 4 or more volts is applied across the LC. EO curve 101 indicates that, when approximately 4.0 or more volts is applied across the LC, essentially no s-polarized light is produced thereby. The electro-optical response of the LC is such that the blocking ratio for s-polarized light is about −40 dB, meaning that only about 0.01% of incident light transmitted through the LC is converted to s-polarized. EO curve 102 indicates that essentially all the incident light is transmitted as p-polarized light when about 4.0 or more volts is applied across the LC. Hence, across a wide voltage range, the LC has a very high blocking ratio when a potential difference greater than about 4.0 volts is applied thereacross.
Conversely, to rotate the polarization of incident p-polarized light 90°, a potential difference of less than about 1.2 volts is applied across the LC. EO curve 101 indicates that, when approximately 1.2 volts or less is applied across the LC, most light transmitted therethrough is s-polarized, i.e., the attenuation of s-polarized light approaches zero. However, EO curve 102 also indicates that the LC has a high blocking ratio of −40 dB for p-polarized light only over a narrow range of control voltage, which is essentially between 1.12-1.20 volts. When a potential difference of less than about 1.12 volts is applied across the LC, attenuation of p-polarized light may be as little as about −20 dB, and up to about 1% of the incident light may be transmitted with no change in polarization. Therefore, even a small variation or drift in applied control voltage, e.g., ±0.1 V, will result in a significant portion of incident light to be transmitted by the LC as p-polarized light. Because an optical steering element subsequently directs a light beam from the LC to different output ports based on the polarization state of the light beam, an unacceptably large portion of the light beam may be directed to an inactive output port when the LC has a low blocking ratio.
In an ideal 1×2 switching operation, an input beam is directed along one optical path and is completely blocked from a second optical path, so that none of the input beam is directed to an inactive output port. This avoids cross-talk, erroneous signals, and other optical network issues. Therefore, optical switches having a high extinction ratio are desirable in optical communications systems. Extinction ratio, which is typically expressed as either a fraction or in dB, is defined as the ratio of optical energy directed to an active output port to the optical energy directed to an inactive output port. Preferably, the optical power level directed to an active output port is three or four orders of magnitude greater than the optical power level directed to an inactive port, i.e., the optical power directed to the inactive port is attenuated −30 to −40 dB.
While LC-based optical switches have advantages over mechanical optical switches, the low blocking ratios inherent with LCs can result in poor extinction ratio for an LC-based optical switch. To reliably improve the extinction ratio of LC-based optical switches, LCs are positioned in series to perform multiple conditioning steps on an input beam, thus compounding the effective blocking ratio of an optical switch. In this way, even when a first LC only has a blocking ratio of −20 dB for light in an unwanted polarization state, a second LC can condition the input beam a second time to produce an effective blocking ratio of −40 dB for the optical switch. This approach, however, requires two or more LCs for a given input beam, substantially adding to the complexity of an optical switching. For example, it is necessary to fabricate and align twice as many LC cells and implement twice as many control electrodes for these LC cells.
In light of the above, there is a need in the art for a non-mechanical optical switch for use in an optical network capable of performing high-extinction ratio switching of a light beam.